1. Field of the Invention
The present invention relates to a wire bonding method, and more particularly to a thermocompression wire bonding method which uses ultrasonic vibrations.
2. Prior Art
FIG. 3 shows basic structure of a thermocompression wire bonder that uses ultrasonic vibrations.
The bonding head 2 is placed on a X-Y table 1, which moves in the X-Y directions located on a flat plane. A horn 3 that moved up and down and vibrates by a Z axis driver (not shown) is placed on the bonding head 2. A bonding tool 4 is provided at the end of the horn 3, and a wire 6 wound on a wire spool 5 is passed through a clamp 7 and brought to the bonding tool 4.
The bonding process for two bonding points (a first bonding surface 8 and a second bonding surface 9) is shown in FIG. 4.
The X-Y table 1, in FIG. 4, is moved horizontally by X-Y axis driver and the horn 3 is moved down by the Z axis driver so that the bonding tool 4 comes into contact with the first bonding surface 8. Then, the wire 6 is pressed on the first bonding surface after the bonding tool 4 is slightly further moved down. After this, the horn 3 makes ultrasonic oscillations so that ultrasonic vibrations are applied onto the bonding tool 4. Thus, the wire 6 is bonded to the first bonding surface 8.
The bonding tool 4 is then raised and moved horizontally in the X-Y directions and further moved down so that the wire 6 is released. Then, the bonding tool 4 comes into contact with the second bonding surface 9, and the wire 6 is bonded to the second bonding surface 9 in the same manner as is done in the first bonding surface 8.
The clamp 7 is then closed and moved up (in the direction in which the wire is inserted), so that the wire 6 is cut and removed from the second bonding surface 9. Thereafter, the bonding tool 4 is moved up, and the clamp 7 is opened, moved up, closed, and then moved down. Thus, the end of the wire 6 is brought to the tip end of the bonding tool 4.
However, in this prior art method, a sufficient bonding strength for the wire cannot be obtained because the entire surface of the wire 6 is not adequately bonded to the first and second bonding surfaces 8 and 9 if these bonding surfaces are in a poor bonding condition due to oxide, etc. accumulated on the bonding surfaces.